The invention relates to a method for generating extreme ultraviolet radiation and soft x-ray radiation with a gas discharge operated on the left branch of the Paschen curve, in particular, for EUV lithography, wherein a discharge chamber of a predetermined gas pressure and two electrodes are used, wherein the electrodes have an opening, respectively, positioned on the same symmetry axis and, in the course of a voltage increase upon reaching a predetermined ignition voltage generate a plasma located in the area between their openings, which plasma is a source of the radiation to be generated, wherein an ignition of the plasma is realized by affecting the gas pressure and/or by triggering, and wherein, with the ignition of the plasma, an energy storage device supplies by means of the electrodes stored energy into the plasma.
A method with the aforementioned method steps is known from DE-A-197 53 696. The method is carried out in a device comprising an electrode system forming the discharge chamber. By means of this electrode system, extreme ultraviolet radiation and soft x-ray radiation are generated that can be used, in particular, for EUV lithography. The electrode system is comprised of two electrodes, i.e., a cathode and an anode, each having an opening. The opening is essentially a hole, and both openings are positioned on a common axis of symmetry. The cathode is embodied as a hollow cathode, i.e., it has a cavity. This cavity is used in order to generate the electrical field in a predetermined way. In particular, the arrangement of the electrodes is such that the field lines in the area or the bore holes are sufficiently stretched so that the firing condition of above a certain voltage is fulfilled. The discharge chamber is filled with gas, and the gas pressure, at least in the area of the electrode system, is within the range of 1 Pa to 100 Pa. The geometry of the electrodes and the gas pressure are selected such that the desired ignition of the plasma is realized on the left branch of the Paschen curve and, as a result of this, no dielectric firing between the electrodes outside of the openings occurs. As a result of the ignition, a current-conducting plasma channel of axial-symmetrical shape results in the area of the openings of the electrodes. In addition, current is supplied by means of the energy storage device via this channel. The resulting Lorentz force constricts the plasma. As a result of this constriction effect and because of resistance heating, very high temperatures occur within the plasma and radiation of a very short wavelength is generated. The known device can produce EUV light in the wavelength range of 10–20 nm.
In connection with the method it is important that a switching element between the electrode system and the energy storage device is principally not needed. Accordingly, a low-inductive and effective coupling of the electrically stored energy into the electrode system can be achieved. Poles energies of a few Joules are sufficient in order to trigger current pulses in the range of several kilo ampere up to several 10 kilo ampere. Triggering of the energy coupling into the discharge that is operated in a controlled fashion or by automatic firing is realized by adjustment to a predetermined ignition voltage. The ignition voltage is affected, for example, by the gas composition, the temperature, pre-ionization, electrical field distribution, and other parameters. It can be adjusted according to the Paschen curve by means of the gas pressure of the discharge vessel. The energy storage device must also be charged up to this ignition voltage in order to be able to supply in the case of ignition as much energy as possible into the plasma.
The invention has the object to improve a method comprising the aforementioned method steps such that the radiation yield, i.e., particularly the yield of EUV light, for each pulse is improved as well as the pulse-to-pulse stability of a plurality of sequentially performed discharges that are utilized in the method performed with pulse operation for generating the EUV light.